Biaxially oriented, heat-shrinkable multilayer foils are used in the packaging of food and drink, in particular of perishable foods, such as poultry or fresh meat, and these multilayer foils preferably have an oxygen-barrier layer. Heat-shrinkable multilayer foils have the property of reverting via shrinkage to their initial unoriented dimensions when they are heated as far as their softening point. Biaxially oriented multilayer foils have been stretched not only longitudinally but also transversely in their production process and their shrinkage is preferably at least 35% both longitudinally or in machine direction and transversely, i.e. in non-machine direction.
Biaxially oriented, heat-shrinkable multilayer foils currently used commercially preferably have an external layer composed of an ethylene-vinyl acetate copolymer, if appropriate mixed with a polyethylene, preferably with an LLDPE, and, as oxygen-barrier layer, a layer based on polyvinylidene chloride, since these thermoplastic materials have excellent shrinkage behavior.
A disadvantage with multilayer foils of this type is that their strength is sometimes inadequate, i.e. their puncture resistance is inadequate when the shrink foil is subject to mechanical stress, e.g. during the packaging of foods with protruding bones. In order, therefore, to improve puncture resistance, heat-shrinkable multilayer foils have previously been equipped with an ionomer layer based on an ethylene-acrylic acid copolymer to some extent in the form of sodium salt. However, in order to achieve adequate adhesion between this type of ionomer layer and a barrier layer based on polyvinylidene chloride, it was necessary to introduce a conventional adhesion-promoter layer. However, this measure implies not only higher materials costs and higher production costs for these shrinkable multilayer foils but also implies relatively high thicknesses, and this is rather undesirable for heat-shrinkable multilayer foils. Furthermore, relatively complicated extrusion equipment is needed.
It was therefore an object of the present invention to provide a heat-shrinkable multilayer foil which has improved mechanical strength, in particular has relatively high puncture resistance together with adequate adhesion without use of an adhesion-promoter layer between the ionomer layer which improves strength and the barrier layer, therefore being thinner than a corresponding multilayer foil used commercially.